In the course of exploratory drilling of the earth, rock samples are often collected to investigate subsurface composition and characteristics. The samples may be collected from various depths of from hundreds to thousands of meters. Such samples are typically collected utilizing core barrel assemblies that include double core tubes having an inner core tube and an outer core tube. While the outer tube may extend through substantially the entire hole, the inner tube may be relatively short, such as on the order of a few meters. The sample is typically collected in the inner tube, which may have a length of a few meters.
In preparation for drilling, an inner tube is inserted into the outer tube until it reaches the bottom of the outer tube so that drilling can begin. A drilling fluid, such as water, used to flush drilling debris from the hole and the tubes may be utilized to exert a force to advance the inner tube through the outer tube. When the inner tube has reached the correct position, a latching mechanism immobilizes the inner tube with respect to the outer tube.
When the inner tube contains the desired sample, the inner tube and sample may then be removed from the hole by attaching a retrieval mechanism to an end of the inner tube assembly. The retrieval mechanism may be suspended on a wire and lowered into the drill string to retrieve the inner tube with the sample. Such an assembly is known as a wireline system. The retrieval mechanism engages an attachment mechanism on the inner tube. The retrieval mechanism then withdraws the inner tube and the sample from the outer tube.
The retrieval mechanism, typically known as an overshot, includes a gripping structure to grip the inner tube. The gripping structure typically includes a claw or “spearhead” to engage a gripping structure in or on the upper end of inner tube of the core barrel. The structure on the inner tube is typically referred to as the head assembly.
To remove the overshot and inner tube containing the sample, force is applied to the wire to pull on the overshot. As force is applied to the wire, the latches are retracted so as to disengage from the walls of the outer tube and the overshot, inner tube and sample are withdrawn from the hole. The overshot may also be utilized to lower a new inner tube into the hole.
A core barrel head and overshot may be utilized when drilling above or below ground. In an underground application, the hole may extend in an upward direction. When exploration and ore definition drilling are carried out in such a context, the core barrel head and overshot typically include at least one sealing member that may be propelled through the drill string with pressurized fluid.
A number of problems may occur with existing designs of core barrel assemblies and overshot assemblies. For example, if the ground formation breaks or fractures, the broken rock may cause the inner tube to become stuck in the outer tube. Typically, freeing the inner tube involves increasing a withdrawing force on the wire. This may cause the wire to break or the hoist to stall. As a result, the entire drill string may need to be removed from the hole to clear the stuck inner tube and make it possible to continued drilling.
To address this problem, the latches have been modified to have a mechanical advantage to unlatch the latches when pulling on the wire line. However, the increased force on the latches may cause the latch linkage system to quickly wear out, resulting in failure of parts. Alternatively, the linkage system will over travel and lock the latches engaged with the outer tube. Additionally, to facilitate the latch linkage, the latches may be made thinner. Reducing the thickness of the latches weakens them, making them prone to break during handling outside of the drill string.
A further problem with known designs of core barrel assemblies is that latches and components may be attached using spring pins. These pins may lose their plasticity over time and may eventually fall out of the assembly unnoticed. Such assemblies can be complex and time consuming to rebuild. Not only does it take time and money to make repairs, but any time a drilling assembly is not in use may result in lost revenues.
Some problems with existing core barrel designs relate to modifications to assist the functioning. For example, to ensure the head assembly turns with the drill string, the outer tube locking surface may include a protrusion to interact with the latches. If the head assembly lands with the latches aligned with the protrusion, the latches will not engage properly. If drilling were to begin in this configuration, the core would not enter the inner tube. This typically results in aborting the drilling of the hole because the inner tube will not be able to capture the suspended core.
Other problems do not relate to the actual latches but rather to other elements. For example, fluid pressure may be utilized to propel the inner tube assembly. To make this possible, sealing devices must be installed on the head assembly. Typically, such sealing devices are installed on the latch retracting unit with a secondary valve. This may increase the number of parts, complexity, and length of the head assembly. If the drilled hole reaches an underground water reservoir, the pressure of the released water can act on the reverse side of the seals activating the latch retracting unit of the head assembly, thereby causing it to disengage with the outer tube and exit the drill string uncontrolled. One solution is this problem is to have a replaceable or permanent assembly between the upper and lower bodies of the head assembly to hold the seals. However, including such an assembly increases the length and requires different parts for the outer tube.
As described above, a core barrel may be utilized to drill in an upward direction. Known head assembly designs typically include a latch locking system to prevent the core barrel head from accidentally disengaging the locking coupling. The locking system typically must be disengaged in order for the operator to insert the latch portion of the head assembly inside the drill string. To disengage the locking system, a short tubular part may be utilized to hold the latches disengaged while the inner tube assembly is pushed into the drill string. The short tubular part slides off of the head assembly when the head assembly is inserted. The short tubular part then falls to the ground. Alternatively, a latch locked head assembly may be inserted by pulling and holding the retracting case in an unlatched position while pushing in the inner tube assembly using only the small spear head. This becomes increasingly difficult the closer a hole is to vertical.
Some problems with existing designs relate to the connection between the overshot and the core barrel assembly. For example, the overshot used to retrieve the inner tube assembly typically includes spring loaded lifting dogs to connect with the head assembly. When the head assembly is lifted out of the drill string, it is often above the operators' heads and is a falling hazard. If the lifting dogs accidentally hit a protrusion on the drill's mast, the overshot can release the inner tube assembly causing injury to the operators.
One method to lock the overshot engaged requires the use of a manually operated lock. Often, it is forgotten to engage the lock. Additionally, the drilling process must be paused to engage the lock, thereby reducing productivity.
Another method to lock the lifting dogs is to nest them within the body of the overshot while the overshot carries the weight of the inner tube. This configuration results in the need to lift the weight of the inner tube in order to disengage the lock. Locking dogs may also be used to prevent the lifting dogs from moving once the lifting dogs are engaged with the head assembly. The locking dogs may be activated by the spearhead entering the overshot body, but it is often engaged prematurely when the over shot is lowered and enters the water remaining in the drill string. This causes the overshot to be locked before reaching the head assembly and locking out the spear head.
Other issues exist relating to the interaction between the latches and the overshot. For example, the gripper on the head assembly to connect with the overshot may include a spearhead point 101 on the top end of the head assembly. If a hole is being drilled upwardly, the spearhead is pointed down toward the operators. If fluid pressure is lost while the inner tube assembly is being pumped through the drill string, gravity will accelerate the inner tube assembly to drop out of the hole toward the operators. An inner tube assembly exiting the drill string uncontrolled can impale an operator causing injury or death.
To address this situation, the spearhead point has been made flexible to ease its handling on the surface. However, the flexible joint often fails to hold the point centered in the drill string, thereby causing the overshot to miss and not engage. Once this happens, rods must be removed to retrieve the inner tube assembly and continue drilling.